Mechano-chemical polishing method for GaAs wafer

ABSTRACT

A method of performing mechano-chemical polishing serving as a primary polishing operation for a GaAs wafer, by using a mechano-chemical polishing solution containing dichloroisocyanuric acid, sodium tripolyphosphate, sodium sulfate, sodium carbonate, and colloidal silica as components except for water, includes the steps of: mounting the wafer on a mechano-chemical polishing apparatus; performing first-stage polishing by supplying the polishing apparatus with the polishing solution having a first composition in which 20-31 mass % of sodium tripolyphosphate is contained in the components except for water; and subsequently performing second-stage polishing by supplying the polishing apparatus with the polishing solution having a second composition in which 13-19 mass % of sodium tripolyphosphate is contained in the components except for water.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polishing method for a GaAs wafer,and particularly relates to an improvement in a mechano-chemicalpolishing method performed as primary polishing for the same.

2. Description of the Background Art

As is well known, crystalline wafers of GaAs that is a Group III-Vcompound semiconductor are utilized as substrates for manufacturingvarious types of semiconductor devices such as a light-emitting elementand a light-receiving element. In recent years, there is an increasingdemand for GaAs wafers used for semiconductor devices particularly inthe field of radio communication, and thus it is desired to increase theproduction efficiency (speedup) and reduce the cost for the same.

A GaAs wafer is cut from a GaAs crystal ingot with a wire saw, a sliceror the like, and trimmed by mechanical grinding or the like. Such a GaAswafer prepared by machining is finally polished in order that its mainsurface is finished into a mirror-like surface.

For polishing of the GaAs wafer, there are generally performed primarypolishing (rough polishing) by mechano-chemical polishing and secondarypolishing (mirror finish polishing) by chemical polishing (e.g., seeJapanese Patent Laying-Open No. 2002-18705 and Japanese PatentLaying-Open No. 2005-264057). In this case, the primary polishing asrough polishing refers to mechano-chemical polishing that is performedutilizing both of abrasive grains and chemical polishing solution, whilethe secondary polishing as mirror finish polishing refers to chemicalpolishing that only utilizes chemical polishing solution withoututilizing abrasive grains.

In other words, an object of the mechano-chemical polishing as theprimary polishing is to increase flatness and smoothness of the waferprepared by the above-described machining. In contrast, an object of thechemical polishing as the secondary polishing is to finish a mainsurface of the wafer, which has its flatness and smoothness improved bythe primary polishing, into a mirror-like surface.

In view of the difference between the objects of the primary polishingand the secondary polishing, the mechano-chemical polishing serving asthe primary polishing is required to have ability to polish at a higherrate than that of the chemical polishing serving as the secondarypolishing. In general, however, if the polishing rate is increased inthe mechano-chemical polishing as the primary polishing, flatness andsmoothness of the wafer tends to relatively deteriorate.

Here, the primary polishing utilizes abrasive grains as well as chemicalpolishing solution, and hence can also cause an effect of trimming thewafer to a certain extent. However, the secondary polishing onlyutilizes chemical polishing solution without utilizing abrasive grains,and hence can cause almost no effect of trimming the wafer. Accordingly,if the mechano-chemical polishing as the primary polishing fails toensure prescribed flatness and smoothness, the chemical polishing as thesecondary polishing cannot achieve desired flatness and mirror-likesmoothness.

Incidentally, for polishing solution usable in the mechano-chemicalpolishing as the primary polishing for a GaAs wafer, there is known,e.g., mechano-chemical polishing solution that containsdichloroisocyanuric acid, sodium tripolyphosphate, sodium sulfate,sodium carbonate, and colloidal silica, except for water (e.g., seeJapanese Patent Laying-Open No. 2005-264057 and Japanese PatentLaying-Open No. 11-283943).

As described previously, there is the increasing demand for GaAs wafersused for semiconductor devices, and hence it is desired to achievespeedup and cost saving of the polishing process for GaAs wafers.

However, if an attempt is made to increase chemical reactivity in thechemical polishing as the secondary polishing to thereby reduce thepolishing time, the wafer surface tends to be roughened and it becomesdifficult to obtain a mirror-like surface. In other words, the chemicalpolishing as the secondary polishing is required to be polishing by agentle chemical reaction so as to achieve a mirror-like surface of thewafer. It is thus inherently difficult to reduce the polishing timethereof.

Accordingly, in order to achieve speedup of the GaAs wafer polishing, itis desired to increase the rate of the primary polishing effected by themechano-chemical polishing. However, if an attempt is made toindiscriminately increase the rate of the primary polishing effected bythe mechano-chemical polishing, prescribed flatness and smoothnesscannot be ensured after the primary polishing, and it becomes difficultto obtain a mirror-like surface of the wafer in the chemical polishingas the secondary polishing.

SUMMARY OF THE INVENTION

In view of the foregoing, an object of the present invention is to makeit possible to increase the polishing rate and ensure flatness andsmoothness of the GaAs wafer in the mechano-chemical polishing as theprimary polishing for the wafer and then to finish the wafer to have amirror-like surface by the chemical polishing as the subsequentsecondary polishing.

According to the present invention, a method of performingmechano-chemical polishing as a primary polishing operation for a GaAswafer, by using mechano-chemical polishing solution containingdichloroisocyanuric acid, sodium tripolyphosphate, sodium sulfate,sodium carbonate, and colloidal silica as components except for water,includes the steps of: mounting the wafer on a mechano-chemicalpolishing apparatus; performing first-stage polishing by supplying thepolishing apparatus with a polishing solution having a first compositionin which 20-31 mass % sodium tripolyphosphate is contained in thecomponents except for water; and subsequently performing second-stagepolishing by supplying the polishing apparatus with another polishingsolution having a second composition in which 13-19 mass % sodiumtripolyphosphate is contained in the components except for water.

It is preferable that the mechano-chemical polishing solution contains40-50 mass % colloidal silica in the components except for water.

Furthermore, it is preferable that, in order to eject the polishingsolution having the first composition from the mechano-chemicalpolishing apparatus after the first-stage polishing, themechano-chemical polishing apparatus is driven for a cleaning period of5-60 seconds after termination of supply of the polishing solutionhaving the first composition, and the second-stage polishing issubsequently performed. Additionally, it is more preferable that wateror water containing the colloidal silica is supplied to the polishingapparatus during the cleaning period.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cross-sectional view of a polishing apparatusused in experiments of mechano-chemical polishing for GaAs wafers in thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As described previously, it is known that polishing solution containingdichloroisocyanuric acid, sodium tripolyphosphate, sodium sulfate,sodium carbonate, and colloidal silica as components except for water isusable for the mechano-chemical polishing as the primary polishing forthe GaAs wafer, (e.g., see Japanese Patent Laying-Open No. 2005-264057and Japanese Patent-Laying-Open No. 11-283943).

As to such mechano-chemical polishing solution, the polishing rate ofthe GaAs wafer and also the flatness and smoothness of the polishedwafer are of course significantly influenced depending on the ratios ofthe components except for water.

Accordingly, the present inventors variously changed the componentratios except for water in the mechano-chemical polishing as the primarypolishing that utilizes the polishing solution containingdichloroisocyanuric acid, sodium tripolyphosphate, sodium sulfate,sodium carbonate, and colloidal silica as components except for water,in an attempt to increase the polishing rate without deterioration inpolished state of the GaAs wafer.

However, in various experiments conducted by the present inventors inmechano-chemically polishing the GaAs wafers while variously changingthe component ratios except for water, increase in polishing rateinevitably caused deterioration in the polished state (flatness andsmoothness of the wafer). It was thus found to be difficult tosignificantly increase the polishing rate without deterioration in thepolished state only by adjusting the composition of the mechano-chemicalpolishing solution:

As a result of the present inventors' studies, it was determined that,in view of the trade-off relation between the rate of the primarymechano-chemical polishing and the demand that the GaAs wafer after theprimary polishing should have flatness and smoothness enough to befinished to have a mirror-like surface in the secondary chemicalpolishing, it was most desirable to contain 23% dichloroisocyanuric acid(“%” will hereinafter be referred to as mass percentage in the presentspecification), 16% sodium tripolyphosphate, 3% sodium carbonate, 8%sodium sulfate, and 50% colloidal silica as component ratios except forwater.

Furthermore, it was found that sodium tripolyphosphate in themechano-chemical polishing solution containing such constituents hadless effect on flatness and smoothness of the GaAs wafer and had thegreatest effect on the polishing rate.

However, it was also revealed that, from the viewpoint of flatness andsmoothness of the GaAs wafer after the mechano-chemical polishing, anacceptable range in variation of the component ratio of sodiumtripolyphosphate was 16%±3% (i.e., 13-19%) and then the adjustment ofthe component ratio of sodium tripolyphosphate within such a range couldprovide only slight increase in polishing rate.

The present inventors, therefore, considered performing themechano-chemical polishing as the primary polishing for the GaAs waferin two stages. More specifically, the present inventors considered thatan object of the first-stage mechano-chemical polishing was tosignificantly increase the polishing rate even though flatness andsmoothness of the GaAs wafer deteriorated within a prescribed acceptablerange, in comparison with the second-stage mechano-chemical polishing.

In contrast, the present inventors considered using, in the second-stagemechano-chemical polishing, the most desirable mechano-chemicalpolishing solution described above, which contains 23%dichloroisocyanuric acid, 16% sodium tripolyphosphate, 3% sodiumcarbonate, 8% sodium sulfate, and 50% colloidal silica as componentratios except for water, or similar mechano-chemical polishing solutionsdifferent in only that the variation range of the component ratio ofsodium tripolyphosphate is 16%+3%.

The present inventors considered that the prescribed acceptable range inwhich flatness and smoothness of the GaAs wafer deteriorates after thefirst-stage mechano-chemical polishing should fall within a range thatcan be corrected by the second-stage mechano-chemical polishing.

Based on the above-described consideration of the present inventors,they conducted a series of experiments in polishing of the GaAs wafer,while variously changing the component ratios of the mechano-chemicalpolishing solution used in the first stage of the two-stagemechano-chemical polishing.

FIG. 1 shows a schematic cross-sectional view of a mechano-chemicalpolishing apparatus used in such experiments in polishing of GaAswafers. In the polishing apparatus, an abrasive cloth 11 is fixed on alower surface of an upper abrasive pad 1. Similarly, an abrasive cloth12 is fixed on an upper surface of a lower abrasive pad 2. GaAs wafers3, which are to be subjected to mechano-chemical polishing, aresandwiched and held between upper abrasive cloth 11 and lower abrasivecloth 12 under a prescribed pressure. A mechano-chemical polishingsolution 4 is supplied to the surfaces of GaAs wafers 3 and abrasivecloths 11 and 12. Upper abrasive pad 1 is made to rotate in a directionof an arrow R1, while lower abrasive pad 2 is made to rotate in adirection of an arrow R2 that is opposite to the arrow R1 direction. Thesurfaces of GaAs wafers 3 are thereby rubbed and polished with abrasivecloths 11 and 12 impregnated with mechano-chemical polishing solution 4.

In the various experiments in polishing conducted by the presentinventors using the mechano-chemical polishing apparatus as shown inFIG. 1, an unwoven polyurethane cloth was used as material of theabrasive pads. The rotational rates of the polishing surface plates were7.7 rpm for the upper surface plate and 23.2 rpm for the lower surfaceplate. The initial dimensions of the GaAs wafers to be polished were 100mm in diameter and 0.6 mm in thickness. For material of abrasive cloths11 and 12, there was used an unwoven cloth made of a resin base materialimpregnated with polyurethane. The supply rate of the mechano-chemicalpolishing solution was 800 ml/min. A load of 50 g/cm² was imposed onGaAs wafers 3, as a pressure at which the wafers were sandwiched betweenabrasive cloths 11 and 12.

Under the conditions of the mechano-chemical polishing apparatus asdescribed above, experiments in the two-stage mechano-chemical polishingwere conducted with various mechano-chemical polishing solutionsadjusted in composition. The results thereof are listed and shown inTable 1 below.

TABLE 1 Component Ratios of Mechano-chemical Polishing SolutionEvaluation After Polishing (Mass %) Two-Stage Polishing SolutionDichloroisocyanuric Sodium Sodium Sodium Colloidal Polishing RateExternal No. Acid Tripolyphosphate Sulfate Carbonate Silica (μm/min)Appearance 1 23% 16% 3% 8% 50% 0.4 ◯ 2 26% 9% 1% 9% 55% 0.2 ◯ 3 24% 16%1% 8% 51% 0.3 ◯ 4 22% 20% 2% 8% 47% 0.6 ◯ 5 21% 21% 4% 8% 46% 0.6 ◯ 622% 22% 2% 8% 47% 0.7 ◯ 7 21% 23% 4% 7% 45% 0.8 ◯ 8 21% 23% 2% 7% 46%0.7 ◯ 9 21% 23% 3% 7% 45% 0.7 ◯ 10 21% 24% 3% 7% 45% 0.8 ◯ 11 21% 24% 4%7% 44% 1.0 ◯ 12 20% 25% 4% 7% 43% 1.1 ◯ 13 20% 26% 3% 7% 44% 1.1 ◯ 1421% 26% 2% 7% 44% 1.0 ◯ 15 20% 26% 4% 7% 42% 1.2 ◯ 16 20% 27% 4% 7% 43%1.1 ◯ 17 20% 27% 2% 7% 43% 1.1 ◯ 18 19% 31% 4% 7% 40% 1.2 ◯ 19 19% 32%2% 7% 41% 1.2 X 20 18% 36% 2% 6% 38% 1.3 X

In Table 1, the numbers attached to the polishing solutions representthe types of polishing solutions used in the first-stagemechano-chemical polishing. In these polishing solutions, a polishingsolution 1 is the one that achieves the best result in view of thetrade-off relation between the polishing rate and the polished statewhen the mechano-chemical polishing of the GaAs wafer is performed inone stage as described above. Note that, when the polishing apparatuswas supplied with each of the various polishing solutions shown in Table1, it was simultaneously supplied with 600 ml/min of a solution in whichapproximately 1.0-1.5 kg in total of the constituents excludingcolloidal silica were dissolved in 45 liters of water and with 200ml/min of another solution in which colloidal silica was diluted withwater to one-third of the concentration.

In the experiment conducted by the present inventors, the GaAs wafer wassubjected to the first-stage mechano-chemical polishing for 45 minuteswhile any of the various polishing solutions shown in Table 1 wassupplied to the polishing machine. Subsequently, the polishing machinewas driven for 30 seconds while the supply of the polishing solution wasstopped. Thereafter the second-stage mechano-chemical polishing wasperformed for 15 minutes while polishing solution 1 was supplied to thepolishing machine. In other words, polishing solution 1 was used in thesecond-stage mechano-chemical polishing, regardless of the types ofpolishing solution used in the first-stage mechano-chemical polishing.

Note that the reason why the polishing machine was driven for 30 secondswhile the supply of the polishing solution was stopped between thefirst-stage mechano-chemical polishing and the second-stagemechano-chemical polishing is that switching from the first-stagemechano-chemical polishing solution to the second-stage mechano-chemicalpolishing solution becomes possible within a shorter period of time.

Table 1 shows the results of evaluation on the two-stagemechano-chemical polishing for the GaAs wafers as described above.Specifically, Table 1 shows the polishing rate (μm/min) and externalappearance, as evaluation items for the two-stage mechano-chemicalpolishing. The external appearance includes flatness and smoothness ofthe GaAs wafer, and it was empirically determined through visualinspection whether or not the GaAs wafer fell within a range thatenables the GaAs wafer to have a mirror-like surface by the chemicalpolishing as the secondary polishing. In Table 1, of course, a sign of“O” means that the GaAs wafer has a favorable external appearance, whilea sign of “X” means that the GaAs wafer has an undesirable externalappearance.

In Table 1, the result of evaluation after the two-stagemechano-chemical polishing using polishing solution 1 is the same asthat of the one-stage mechano-chemical polishing using the samepolishing solution, because polishing solution 1 was used in both of thefirst and second stages in the two-stage mechano-chemical polishing.Therefore, it is possible to determine whether or not preferable resultscan be obtained with the two-stage polishing, by comparing theevaluation results after the two-stage mechano-chemical polishing inwhich various polishing solutions were used in the first-stagepolishing, with the evaluation results of the case where polishingsolution 1 was used.

As seen from Table 1, when polishing solution 2 or 3 was used in thefirst-stage mechano-chemical polishing, the GaAs wafer achieved afavorable external appearance after the two-stage polishing. However,the polishing rate thereof was lowered as compared with the case ofpolishing solution 1. It is therefore concluded that intention ofperforming mechano-chemical polishing in two stages becomes meaninglessif polishing solution 2 or 3 is used in the first-stage mechano-chemicalpolishing.

In contrast, when polishing solution 19 or 20 was used in thefirst-stage mechano-chemical polishing, the polishing rate wassignificantly increased as compared with the case of polishing solution1. However, the GaAs wafer obtained after the two-stage polishing had anundesirable external appearance, and thus failed to obtain a mirror-likesurface after the chemical polishing as the secondary polishing. It istherefore concluded that intention of performing mechano-chemicalpolishing in two stages becomes meaningless as well if polishingsolution 19 or 20 is used in the first-stage mechano-chemical polishing.

However, when any of the polishing solutions 4-18 was used in thefirst-stage mechano-chemical polishing, the GaAs wafer achieved apreferable external appearance after the two-stage polishing, and thepolishing rate thereof was clearly increased as compared with the caseof polishing solution 1. In other words, it was found that performingthe two-stage polishing by using any of polishing solutions 4-18 in thefirst-stage mechano-chemical polishing and subsequently using polishingsolution 1 in the second-stage mechano-chemical polishing could clearlyincrease efficiency of the mechano-chemical polishing as the primarypolishing for the GaAs wafer, as compared with the case of the one-stagemechano-chemical polishing using polishing solution 1.

In the first-stage mechano-chemical polishing solutions shown in Table1, the component ratio of sodium tripolyphosphate was varied mostsignificantly among the component ratios except for water. Thiscomponent was found to most dominantly and most systematically influencethe polishing rate of the two-stage polishing and the externalappearance after the two-stage polishing. In other words, it was foundthat the component ratio of sodium tripolyphosphate that is a componentother than water in the first-stage mechano-chemical polishing solutionshould fall within a range from 20% as in polishing solution 4 to 31% asin polishing solution 18.

Note that, although polishing solution 1 containing 16% sodiumtripolyphosphate was used in the second-stage polishing of the two-stagemechano-chemical polishing in the various experiments in Table 1, apolishing solution containing sodium tripolyphosphate in the range of16%±3% may also be used in a similar manner.

In the various experiments in Table 1, the polishing apparatus wasdriven for a cleaning period of 30 seconds while the supply of thepolishing solution was stopped between the first and second stages ofthe two-stage mechano-chemical polishing, in order to eject thepolishing solution used in the first stage and clean the inside of thepolishing apparatus. The cleaning period is only required to fall withina range of 5-60 seconds, and preferably falls within a range of 10-40seconds.

In other words, too short a cleaning period is not preferable becausethe polishing solution used in the first stage remains even in thesecond stage of the two-stage mechano-chemical polishing and then theGaAs wafer tends to have an undesirable external appearance. Incontrast, driving the polishing apparatus for an excessively longcleaning period in a state of no supply of the polishing solution is notpreferable because the wafer surface tends to have traces of having berubbed by the abrasive cloth or the wafer tends to have cracks.

As is understood from the foregoing, it is more preferable, in thecleaning period between the first and second stages of the two-stagemechano-chemical polishing, to drive the polishing apparatus with nosupply of the mechano-chemical polishing solution and with supply ofwater or water containing colloidal silica.

As described above, according to the present invention, by performingtwo-stage polishing in the mechano-chemical polishing serving as theprimary polishing for the GaAs wafer, it is possible to achieve speedupand cost saving in production of the GaAs wafers, which should then bepolished to have their mirror surfaces by the chemical polishing servingas the subsequent secondary polishing, and to respond to the increasingdemand for the mirror-finished GaAs wafers.

Note that, although the wafers having a diameter of 100 mm (4-inchdiameter) have been described in the above-described examples, thepresent invention may of course be applied to wafers having any otherdiameters.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the scopeof the present invention being interpreted by the terms of the appendedclaims.

1. A method of performing mechano-chemical polishing serving as aprimary polishing operation for a GaAs wafer, by using amechano-chemical polishing solution containing dichloroisocyanuric acid,sodium tripolyphosphate, sodium sulfate, sodium carbonate, and colloidalsilica as components except for water, comprising the steps of: mountingsaid wafer on a mechano-chemical polishing apparatus; performingfirst-stage polishing by supplying-said polishing apparatus with apolishing solution having a first composition in which 20-31 mass %sodium tripolyphosphate is contained in said components except forwater; and subsequently performing second-stage polishing by supplyingsaid polishing apparatus with another polishing solution having a secondcomposition in which 13-19 mass % sodium tripolyphosphate is containedin said components except for water.
 2. The method of mechano-chemicalpolishing for the GaAs wafer according to claim 1, wherein saidmechano-chemical polishing solution contains 40-50 mass % colloidalsilica in the components except for water.
 3. The method ofmechano-chemical polishing for the GaAs wafer according to claim 1,wherein, in order to eject the polishing solution having said firstcomposition from said polishing apparatus after said first-stagepolishing, said polishing apparatus is driven for a cleaning period of5-60 seconds after termination of supply of the polishing solutionhaving said first composition, and said second-stage polishing issubsequently performed.
 4. The method of mechano-chemical polishing forthe GaAs wafer according to claim 3, wherein water or water containingthe colloidal silica is supplied to said polishing apparatus during saidcleaning period.